calculated two new rasters with a threshold of 100 and 500 cells, respectively

converted each stream raster into a polyline feature.

What bothers/surprises me is that many of these polylines are unnaturally straight. I expected a much more irregular pattern. I have actually been in the field and seen the area. There are no artificial channels that could explain why the lines are so straight.
Is there any mistake in my calculation process? Any step I missed? Have you ever had a similar output when using the flow accumulation tool? Or would you say that the result makes sense?